1. Field of the Invention
The present invention relates to an image display method and its apparatus capable of obtaining an image of high picture quality on a display screen of, for example, television signal.
2. Related Art of the Invention
The television screen size is becoming larger recently, and the projection type system using a liquid crystal panel is highly expected for a large screen over 40 inches.
At the same time, broadcasting of high definition television has begun, and there is a mounting need for a display device of high resolution and wide area.
Further, by three-CCD camera and others, image pickup of high dynamic range is realized.
However, the display device of high resolution and wide area is not sufficient in the contrast performance, and the image of high dynamic range cannot be expressed sufficiently by a conventional display device.
As its improving method, a technique as disclosed in Japanese Laid-open Patent 7-170428 (Japanese Patent Application 6-3457) has been proposed.
This conventional contrast improving method is described below.
FIG. 13 is a block diagram of its entire apparatus, in which reference numeral 101 is a gradation conversion circuit for converting the gradation of input luminance, and 102 is an illusion waveform generating circuit for generating a waveform for causing an illusion from the input luminance, and this apparatus is designed to obtain an image improved in contrast by superposing an illusion waveform on the luminance converted in gradation by the gradation conversion circuit 101.
The detail of the gradation conversion circuit 101 and illusion waveform generating circuit 102 is described below. FIG. 14 is a block diagram of the gradation conversion circuit 101. A frame memory 123 stores the luminance input for the portion of one frame. An edge extraction circuit 124 extracts an edge from the luminance input. A gradation conversion table calculating circuit 125 divides one frame into 9 regions shown in FIG. 15, and calculates the gradation conversion tables in nine representative points q1 to q9 of nine regions according to a predetermined rule, by making use of the luminance of the pixels of the edge neighbor regions in each divided region, and writes them into a memory table 126.
A gradation conversion table interpolation circuit 129 reads coordinate values of an arbitrary element of notice to be converted in gradation, from a read control circuit 127, interpolates the gradation conversion tables in the representative points q1 to q9 corresponding to the coordinate values (for example, linear interpolation for weighting and adding the gradation conversion tables at four neighbor points, by the distance between the four points and element of notice), and writes into a table memory 128.
On the other hand, the read control circuit 127 reads out the luminance data of pixels in the sequence of scanning from the frame memory 123, and issues the read coordinate values of the pixel into the gradation conversion table interpolation circuit 129, and reads out the converted value of the luminance of the pixel from a table memory 128, and issues as a converted luminance.
Next, the illusion waveform generating circuit 102 is described. FIG. 16 shows a step input into the illusion waveform generating circuit 102, and FIG. 17 shows a step response of the illusion waveform generating circuit 102.
In FIG. 16 and FIG. 17, for the sake of simplicity, the luminance distribution is expressed one-dimensionally. The step response of the illusion waveform generating circuit 102 attenuates the low frequency components of the step input, and converts the output characteristic (gain characteristic) into a luminance amplitude range not causing feel of strangeness at the time of observation.
FIG. 18 shows a luminance distribution sensed at the time of observation, by superposing the step response of illusion waveform generating circuit 102 on the step input of the original input. In FIG. 18, a solid line shows the luminance level of the step response of the illusion waveform generating circuit 102 superposed on the step input, and a broken line refers to the luminance distribution sensed at the time of observation. As understood from this broken line, FIG. 18 shows that the psychological contrast is improved by superposing the output of the illusion waveform generating circuit 102.
Furthermore, concerning the luminance distribution as indicated by solid line in FIG. 19, a luminance difference of I1-I0 is sensed at position x1 as indicated by a broken line, while a luminance difference of I2-I1 is sensed at position x2, and between positions x1 and x2, a luminance difference of I1 and I2 is not sensed, so that the luminance differences I1-I0, I3-I2 greater in sum than the luminance range of I3-I0 can be displayed in the luminance range of. I3-I0, thereby improving the contrast.
Thus, hitherto, by utilizing both local gradation conversion on the basis of the luminance distribution near the edge and the superposition of the illusion waveform, the gradation reproduction capability of the display device is enhanced, and the contrast sensed at the time of observation of image is improved.
Such conventional constitution, however, had the following problems.
1) The boundary of the no-picture area around the image (corresponding to the horizontal and vertical blank period) is extracted as an edge, and the luminance frequency distribution near the boundary (that is, the luminance frequency distribution in the peripheral part of image) has a large affect on the result of processing, and the contrast in the middle of the image is relatively lowered as compared with the peripheral part.
2) When the entire image is divided into nine local regions, the region initially uniform in luminance in the entire screen may be converted bright in the image peripheral part and dark in the central part, or, to the contrary, dark in the peripheral part and bright in the central part, and hence strangeness may be felt when observing.
3) The circuit scale is larger as compared with other conventional techniques (histogram equalization).
4) The circuit scale is too large depending on the manner of generating illusion waveform in the prior art.